crystal oscillator model
You first need to decide on the topology of the oscillator itself. This is dictated by the target complexity and oscillator performance requirements. Then you need to define what problem you are trying to solve by employing an AGC loop (potentially this problem isn't a real problem, or it can be solved in a different way). If AGC is still required, then you need to assess the loop control bandwidth to counter the amplitude effects that limit your target performance.
About a crystal model: it is best that you contact the target crystal manufacturer for that, since there are large variations (related to fund. or overtone, package type and parasitics, the way the crystal is cut, specified loading capacitance, etc.). There doesn't exist any standard crystal model. You really have to target some specific vendors and types.